1. Field of the Invention
This invention relates to thermal transfer materials and, more particularly, to a dry thermal interface material, and related methods of producing and applying the material between an electronic component and a heat sink.
2. Description of the Related Art
Electronic assemblies are usually fabricated with a plurality of electronic components attached to a substrate, such as a printer circuit board. In order for these assemblies to operate properly and reliably for extended periods of time, the heat generated by the components must be efficiently and reliably transferred from the component to the board, which acts as a heat sink.
Such electronic assemblies are operating at increasingly higher temperatures as they are built smaller and run faster. With smaller electronic components, the density can also be increased, which further increases the need for efficient and reliable removal of heat.
The ultimate theoretical thermal transfer occurs where a component and the heat sink interface in continuous contact. In reality, however, the respective surfaces of the component and heat sink have irregularities, such as microscopic voids or pores, which trap air. Since air is a poor conductor of heat, these irregularities/voids must be filled with some thermally conductive material, to effect more efficient thermal transfer. The following materials and techniques have been used to promote this thermal transfer.
Silicone-based thermal grease served as an early thermal interface material for electronic assemblies. Such grease is formed by dispersing thermally conductive ceramic fillers in silicone to form a sticky paste.
When the grease is applied between a surface of the electronic component and a surface of the heat sink, the grease fills the voids and eliminates the interstitial air. Any excess grease flows out at the edges of the component. The use of this grease allows for the thinnest possible joint as both mating surfaces come into contact at their high points, resulting in a very low thermal resistance.
Although such grease has proved to be a very good thermal conductor, problems are associated with its use. It is messy, due to its moist-to-the-touch, sticky state, and it is time-consuming to apply (e.g., generally the right amount of grease must be applied). Also, if the grease is applied to a protective sheet liner, to facilitate handling, shipping, etc., when the liner is removed prior to application of the grease to the electronic component surface, up to 50% of the grease may remain on the liner, causing waste, increasing costs, and resulting in a less effective thermal interface than desired. In addition, during operation of the electronics, when heat is being generated, the thermal grease migrates away from the area of application. Also, silicon-based greases exhibit the disadvantage of causing silicone contamination of a wave solder bath. If silicone oil migrates onto a printed circuit board, any solder re-work on the board will not adhere. Such migration may also cause short circuits on the board.
Non-silicone thermal grease was then developed to address many of the above-discussed problems associated with silicone-based products. Non-silicone greases are formed by dispersing the thermally conductive ceramic fillers in hydrocarbon oils.
While the non-silicone-based greases addressed the migration/contamination characteristics of silicone-based products, they still suffered from being messy, since they still exhibited moist/sticky characteristics, and they were still difficult and time-consuming to apply.
In a further effort to provide an acceptable replacement for thermal grease, relatively thicker and drier elastomeric thermal pads were developed. Their composition is basically silicone rubber-containing heat-conducting particles, such as zinc oxide, aluminum oxide, aluminum nitride, and boron nitride. The advantages of using these pads have included the facts that they are less messy (due to being drier), installation is easier and less time-consuming, and they eliminate the need to apply only the correct amount of grease with each application.
As noted above, however, the ultimate thermal interface is where two parts touch at as many points as possible, and only where microscopic voids appear, are they filled. Whereas the above-described grease flows easily into these voids, and is easily displaced to allow as much direct contact as possible between the component and the heat sink, these pads do not allow for any direct contact between the surfaces of the component and the heat sink. That is, these silicone elastomers deform to surface irregularities only when a significant compressive load is applied, which may be detrimental to the electronic component. At low pressure, the pad simply cannot fill the air voids between the surfaces, causing a relatively very high thermal resistance.
Wax or paraffin-based phase-change materials have also been developed, which exhibit grease-like thermal performance and, due to their relative dryness, exhibit easier elastomeric pad-like handling and installation. These phase-change materials have been used in a stand-alone form, have been reinforced with fiberglass, or have been coated onto foil or Kapton(copyright). Kapton is a thermally conductive but electrically insulative polyimide film available from the DuPont company. These phase-change materials are solid at room temperature, but they behave much like thermal pastes or greases once they reach their phase-change, or melt operational temperature, i.e., usually between 40xc2x0 C. and 70xc2x0 C.
Since these phase-change materials are solid and dry at room temperature, they are not messy to apply. As they are heated they become liquid and flow into the pores. However, in a vertical orientation of the electronic component, they will flow out of the interface, again leaving voids. These materials require pressure sensitive adhesives to adhere to parts during assembly, which adhesives undesirably increase thermal resistance. The operational high temperature range for phase-change materials is only 150xc2x0 C., however, versus 200xc2x0 C. for thermal grease. Further, in a xe2x80x9ccold platexe2x80x9d application, i.e., using water and/or thermo-electric modules to help cool electronic assemblies, the temperature would not reach the melt operational temperature, so the phase-change material would not receive enough heat to melt into place (wet the surface), and therefore would not be useable, whereas grease works at such temperature. Further, each thermal cycle and subsequent phase-change may introduce new air voids that may not be refilled.
In light of the above, thermal pads are easy to use, but exhibit a relatively high thermal resistance. And, while phase-change materials may outperform pads in terms of thermal transfer efficiency, they still bear limitations in use and performance. Thermal grease offers superior performance to these grease replacements, including most particularly the lowest thermal resistance, but can be very messy and labor-intensive during application.
Although the prior art described above eliminates some of the problems inherent in the thermal transfer art, this prior art still does not disclose or teach the most efficient compound and related methods of production and use.
Accordingly, it is a purpose of the present invention to provide a thermal interface material that exhibits the positive attributes of conventional thermal greases, but it easier to apply.
It is another purpose of the present invention to provide a dry thermal interface material that allows for total wetting action to fill any voids between an electronic component and a heat sink, without the need to change the phase of the material.
It is another purpose of the present invention to provide a thermal interface material having a positive coefficient of thermal expansion and thixotropic properties to improve wetting action, thereby facilitating total thermal interface contact between an electronic component and a heat sink.
It is yet another purpose of the present invention to provide a thermal transfer material that allows immediate heat transfer at any operational temperature, without the need for a phase-change, making the material particularly appropriate for cold plate applications.
It is further a purpose of the present invention to provide a thermal transfer material that offers the advantages and conveniences of thermal pads and phase-change materials, but has the superior performance of thermal grease.
It is still another purpose of the present invention to provide a thermal grease which facilitates handling and prevents migration.
It is but another purpose of the present invention to provide a non-silicone- and nonwaxed-based thermal grease or paste that may be naturally tacky.
It is another purpose of the present invention to provide a thermal transfer compound which can be molded into sheets, blocks and other forms, and then cut, to facilitate placement between an electrical component and a heat sink.
It is also a purpose of the present invention to provide a drop-in-place thermal transfer compound that is easy to use and handle in many manufacturing environments.
It is also a purpose of the present invention to provide a thermal transfer compound which can be applied with minimal pressure.
It is still another purpose of the present invention to provide a dry but naturally tacky thermal transfer material that does not require any adhesive or other additives which might reduce thermal transfer efficiency.
It is also a purpose of the present invention to provide a thermal transfer compound that is thixotropic in nature to prevent run out from between an electronic component and a heat sink during operation of the component.
It is another purpose of the present invention to provide a method for forming a relatively dry thermal transfer compound, but with some tackiness.
It is also a purpose of the present invention to provide a thermal transfer material that exhibits both superior thermal transference and electrical insulative properties.
It is another purpose of the present invention to provide a method for more effectively applying a thermal transfer material between an electronic component and a heat sink.
It is also a purpose of the present invention to provide a method for applying a thermal transfer material using only a minimal amount of force to effect total interface contact between an electronic component and a heat sink.
To achieve the foregoing and other purposes of the present invention there is provided a thermal transfer material including a compound that has high thermal conductivity, is relatively dry-to-the-touch, is naturally tacky, and may be formed into various shapes, such as blocks, sheets, etc., to facilitate its application between an electronic component and a heat sink. The compound includes a pre-blend made up of a polyol ester, and an antioxidant, as well as a filler(s), a high viscosity oil, and either a polystyrene-based polymer, a solvent, and a surfactant, or aluminum silicate.
The present invention is also directed to providing a method for producing the compound including the steps of mixing polyol ester in an amount of about 99 wt. percent and an antioxidant in an amount of about 1 wt. percent to form the pre-blend (which pre-blend makes up about 8-12 wt. percent of the compound), adding at least one of a zinc oxide filler in the amount of about 18-80 wt. percent and a magnesium oxide filler in the amount of about 60 wt. percent to the pre-blend; and adding a high viscosity oil in the amount of about 2.5-5.5 wt. percent. Further, in one embodiment there is added a surfactant in the amount of about 0.2 wt. percent, a polystyrene-based polymer in the amount of about 3 wt. percent, and a solvent in the amount of about 1 wt. percent. In an alternate embodiment, instead of the polymer, solvent and surfactant, there is added aluminum silicate in an amount of about 5.2 wt. percent of the compound.
This dry-to-the-touch thermal transfer material offers very low thermal resistance at lower closure pressure, like conventional thermal grease, but offers the handling ease of the conventional grease replacements discussed above, thereby eliminating the need to sacrifice thermal performance for convenience. The block, sheet, etc. forms of the material can be die-cut and possess a natural tackiness that allows them to adhere to an electronic component or heat sink without using additional adhesives that would degrade thermal performance. The material also exhibits a positive coefficient of thermal expansion and exhibits thixotropic properties which allow it to wet surfaces, further improving interface contact. And, because heat transfer begins immediately and can take place at any temperature, it is excellent for cold plate applications. The material is also silicone free to avoid problems of silicone contamination, and can be electrically insulating, if desired.
The present invention is also directed to a method for providing a thermal interface material for electronic component assemblies, including the following steps: providing a heat generating electronic component with a first mounting surface; providing a second mounting surface on a heat sink upon which the first mounting surface of the heat generating electronic component is to be mounted; and disposing the dry thermal transfer material discussed above between the first mounting surface and the second mounting surface to effect heat transfer from the electronic component to the heat sink.
Further, the material can include a thermally conductive foil backing, or a thermally conductive and electrically insulative backing, and be die cut, if desired. Also, removable liners can be applied to exposed surfaces of the compound to facilitate handling, shipping and storage, but same are removed prior to the material being applied between the electronic component and the heat sink.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.